The use of salt in freshwater tanks has recently been a controversial
subject of some debate. This article is intended to give some objective
information so that the beginner can make their own decision.

First off, some definitions. When we speak of "salt", this is a confusing
term, because salts are a large heterogeneous group that consists of
any ionically bound elements or compounds. The salt that most freshwater
aquarists mean to speak of, however, is NaCl (sodium chloride). THIS
SHOULD NOT BE CONFUSED WITH MARINE SALT, WHICH CONTAINS BUFFERS THAT
ARE VERY HARMFUL TO MANY FRESHWATER FISH. Table salt is basically also
NaCl, only with iodine added (the myth that iodized salt is harmful
fish is largely false... iodine is a natural element that is needed
in the diet of all fish, and is actually a supplement in many fish foods...
the only thing in table salt of real harm to FW fish are the decaking
agents, which are very low in concentration).

********************PROS*******************

The reasons that some aquarists use salt in their FW tanks (usually
at a concentration of 1 tablespoon / 5 gallons... do not confuse this
with "brackishness" as this is a much lower concentration, and true
brackish tanks use mostly other types of "hard" salts... see warning
against marine salt, above) as a general preventative to disease are
as follows:

Salt adds electrolytes, which reduce osmotic stress to the gills.
This is especially important during disease, when the ability of the
fish to maintain homeostasis with its surrounding water is disturbed.

Salt can aid in the production of the slime layer and speed up the
healing of some wound sites. This occurs by hyperosmolarity... fish
have a certain specific gravity (concentration of total dissolved
solids) in their bodily fluids, and so does the surrounding water...
if the surrounding water has a slightly higher concentration, the
fluids from the wound site move into the water around it, and fresh
plasma goes in to replace it, creating more blood flow in that area
(makes it heal faster).

Many pathogens, such as Ichthyophthirius (the protozoan that causes
"ick"), do not particularly like salt.

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********************CONS********************

Despite some of its (rather minor) potential benefits, there are a
lot of potential dangers to consider in using salt, especially if you
are a beginner. Please make special note of these.

Although NaCl is not composed of any truly "hard" ions (laundering
ions of Mg or Ca, which produce the hard water stains on your tank
and are the only ions that count in true carbonate hardness), it does
raise the total dissolved solids in the water... these add up to raise
general hardness, or GH. This is not well tolerated by a number of
fish, especially true softwater fish from places like the Amazon river
basin, where there are very few electrolytes of any kind in the water.
These fish include (but are not limited to) neon tetras, cardinal
tetras, rummy nose tetras, hatchetfish, elephantnoses, and discus...
in addition, most live plants will not tolerate it either. DO NOT
USE SALT WITH ANY OF THESE FISH, AS SOME HAVE RATHER STRINGENT PH/KH/GH
REQUIREMENTS, AND COULD DIE AS A RESULT OF THIS ADDITION. There are
other Amazonian fish such as angelfish which will not particularly
prefer salt, but may possibly tolerate it because they are hardier.
Salt can also have an unpredictable effect on other fish, since there
are no bodies of water in Nature which are naturally saline (high
in NaCl) but very low in "true" hardness ion concentration (Mg, Ca,
etc)... Rift African species, for example, need more than just ordinary
"aquarium salt" (NaCl alone).

Salt, if not predissolved carefully, can give fish bad burns. This
is especially true of scaleless fish, such as some types of eels and
scaleless catfish.

The efficacy (effectiveness) of salt has not been proven experimentally
to the satisfaction of many in the pet trade. Some swear by it, but
many, as you can see, swear against it.

THE GENERAL RULE IN ANY TANK, ESPECIALLY THAT OF A BEGINNER, IS TO
STEER CLEAR OF ADDING ARTIFICIAL ADDITIVES UNLESS ABSOLUTELY NECESSARY.
Salt in many tanks is not... in a livebearer only tank (such as one
with mollies, swordtails, guppies, or platys), I would consider recommending
it (these are moderately hard water fish which like electrolytes in
their water... some, such as the molly, from it's Yucatan peninsula
natural habitat, actually prefer it because they are semi-brackish),
but since so many community tanks contain more delicate, soft water
fish such as the neon tetra, I usually do not. Please consider all your
fish carefully when setting up a tank environment, as harmony in a mini-ecosystem
is extremely difficult to achieve and any one thing can throw it off.
The (bolded) note on chemical additives goes not only for salt addition,
but also for medications, pH up/down solutions, snail/algae eradicator,
etc... if you are a novice and not entirely familiar with what you are
adding, USE GREAT CAUTION, and educate yourself completely on the potential
effects (some of which are unpredictable) of that product before you
add it.